The present invention generally relates to position control apparatuses for controlling positions of rotary heads with respect to a recording medium, and more particularly to a position control apparatus for controlling relative positions of a plurality of rotary heads with respect to a magnetic surface of a magnetic tape, in a recording and/or reproducing apparatus wherein there exists a time period in which two or more rotary heads among the plurality of rotary heads simultaneously scan over the magnetic tape.
Generally, an existing helical scan type recording and reproducing apparatus (VTR) records a video signal by rotary heads on tracks which are formed obliquely to the longitudinal direction of a magnetic tape upon recording, and reproduces the recorded signal from the tape upon reproduction. Amcng this type of a helical scan type VTR, there is a known 4-head type VTR in which four rotary heads are equally spaced apart and mounted on a rotary body such as a rotary drum and a rotary plate, so that adjacent rotary heads are spaced apart by 90.degree.. In this 4-head type VTR, two mutually opposing rotary heads have gaps of the same azimuth angle. In addition, the adjacent rotary heads have gaps of mutually different azimuth angles. Certain conditions must be satisfied in order to form a tape pattern on the tape by the 4-head type VTR, so that the tape pattern is identical to a tape pattern which is formed on the tape by the existing VTR (hereinafter referred to as a 2-head type VTR) which carries out the recording and reproduction by use of two rotary heads which are mounted diametrically on a rotary body. One condition which must be satisfied, is to select the azimuth angles of the gaps in the four rotary heads to the same azimuth angles as the gaps in the two rotary heads of the 2-head type VTR. Another condition to be satisfied, is to wrap the tape around the peripheral surface of the rotary body over an angular range of approximately 270.degree.. Still another condition which must be satisfied, is to select the tape traveling speed so that the tape travels by one track pitch during one field period in which one rotary head rotates by 270.degree..
If the above conditions are satisfied, the length of the tape which is in contact with the peripheral surface of the rotary body in the 4-head type VTR, becomes equal to the length of the tape which is in contact with the peripheral surface of the rotary body in the 2-head type VTR. In addition, the relative linear speed between the tape and the rotary head in the 4-head type VTR becomes equal to the relative linear speed in the 2-head type VTR. As a result, one field of the video signal is successively recorded on one video track by the four rotary heads, and it becomes possible to form a tape pattern on the tape which is identical to the tape pattern formed by the 2-head type VTR. Therefore, a compatible tape pattern is formed so that perfect compatibility can be ensured between the 2-head type VTR and the 4-head type VTR.
Among the four rotary heads in the 4-head type VTR, one field of video signal is recorded on or reproduced from one video track in a time period in which one of the four rotary heads rotates over approximately 270.degree.. During this time period in which one rotary head which is recording or reproducing rotates over approximately 270.degree., a rotary head which lags the one rotary head by 90.degree. in the rotating direction and a rotary head which lags the one rotary head by 180.degree. in the rotating direction successively scan over a part of the one video track. Accordingly, at the time of the recording, it is necessary to successively switch and supply the video signal to only one of the four rotary heads which is to carry out the recording. Further, at the time of the reproduction, it is necessary to successively switch and obtain a reproduced signal from only one of the four rotary heads which is to carry out the reproduction.
However, in the 4-head type VTR, during a time period in which one of the four rotary heads scans over a non-overlap recording section on the magnetic tape, two of the remaining three rotary heads are also in contact with the magnetic tape. On the other hand, during a time period in which the one rotary head scans over an overlap recording section on the magnetic tape, the remaining three rotary heads are also in contact with the magnetic tape, that is, all the four rotary heads are in contact with the magnetic tape. For this reason, during the time period in which one field of the video signal is recorded on one video track on the magnetic tape, each of the rotary heads other than the rotary head which is carrying out the recording, make contact with the magnetic tape once. In other words, during this time period in which one field of the video signal is recorded, a rotary head other than the rotary head which is carrying out the recording, makes contact with the magnetic tape three times in total. Therefore, during this time period in which one field of the video signal is recorded, the magnetic tape receives an impact every time the rotary head other than the rotary head which is carrying out the recording, starts to make contact with and terminates the contact with the magnetic tape. The magnetic tape receives such impact six times during this time period in which one field of the video signal is recorded. Thus, the magnetic tape vibrates every time it receives the impact, and there is a problem in that a so-called impact error occurs.
According to a known foil bearing theory disclosed in a Japanese Laid-Open Patent Application No. 57-94957 or the like, an air film (layer) thickness h between the magnetic tape and the outer peripheral surface of the rotary body which is mounted with the four rotary heads, can be obtained from the following equation (1), where R is the radius of the rotary body, K is a constant, T is the tape tension, .mu. is the coefficient of viscosity of air, and V is the relative speed between the magnetic tape and the rotary head. EQU h=KR(6.mu.V/T).sup.2/3 ( 1)
Thus, the air film thickness h is proportional to the radius R of the rotary body. In the case of the 4-head type VTR, the value of the radius R of the rotary body is 2/3 the value of the radius of the rotary body in the existing 2-head type VTR. Hence, as may be seen from the equation (1), the air film thickness h in the 4-head type VTR is 2/3 the air film thickness in the existing 2-head type VTR. The four rotary heads project from the outer peripheral surface of the rotary body by quantities (hereinafter referred to as head projecting quantities), and the four rotary heads lift the magnetic tape in the shape of a tent or a tepee. For this reason, when the head projecting quantities of the four rotary heads is maintained the same as the corresponding head projecting quantities in the 2-head type VTR, the magnetic tape receives a greater impact when the rotary head makes contact with the magnetic tape. Further, compared to the 2-head type VTR, the impact on the magnetic tape is also greater when the rotary head terminates contact with the magnetic tape and separates from the magnetic tape, because the rotary head lifts the magnetic tape over a greater distance in the case of the 4-head type VTR. This impact on the magnetic tape causes the magnetic tape to vibrate, and results in a time base deviation (jitter) and color phase deviation.
Accordingly, it is necessary to reduce the head projecting quantities in the 4-head type VTR, compared to the corresponding head projecting quantities in the 2-head type VTR. However, when the head projecting quantities are reduced, the distance between the tip end of the rotary head and the outer peripheral surface of the rotary body becomes shorter. As a result, the serviceable life of the rotary head is shortened because the permissible range in which the rotary head is permitted to wear out to the outer peripheral surface of the rotary body is reduced, and moreover, since the time period in which the rotary head is in contact with the magnetic tape is 3/2 times that of the 2-head type VTR. In addition, even when the head projecting quantities are reduced, the magnetic tape still receives the impact when the rotary head makes contact with and terminates the contact with the magnetic tape.
At the time of the recording in the 4-head type VTR, the crosstalk does not become a problem since the video signal to be recorded is switched and supplied to only one of the four rotary heads which is to carry out the recording. However, at the time of the reproduction, two or three rotary heads other than the one rotary head which is to carry out the reproduction, also scan over the magnetic tape at the same time as when the one rotary head scans over the magnetic tape. Hence, there is a problem in that reproduced signals from those rotary heads other than the one rotary head pass through a rotary transformer which is coupled to the one rotary head, and mix into the reproduced signal from the one rotary head as crosstalk.